1. Field of the Invention
The present invention relates to a microwave filter, and more particularly, to a tunable recursive filter employed in a microwave monolithic integrated circuit (MMIC). This application is based on Korean Patent Application No. 97-66765, which is incorporated by reference herein for all purposes.
2. Description of Related Art
A microwave telecommunication system, e.g., cellular phone system, a global positioning system or a satellite broadcasting system, typically requires a small, low-power, high-frequency filter circuit with a high selectivity Q. Such a high-frequency filter circuit can be implemented by a passive circuit using passive components such as resistors, inductors and capacitors, as in a low-frequency filter. The passive filter may be implemented by using distributed circuit elements which are obtained by appropriately connecting a plurality of transmission lines, or by using discrete circuit elements.
When distributed circuit elements are used in the filter on a semiconductor integrated circuit (IC) chip, the transmission lines typically occupy a very large amount of chip area, which results in inefficient utilization of the chip area. Also, in the case of using discrete circuit elements, the resistors, inductors and capacitors used for the filter must be manufactured to have very small size. However, when the passive filter is implemented on a monolithic IC, it is difficult to accurately control resistance, inductance and capacitance in very small components on a semi-insulating substrate to very small, accurate circuit element values. Further, the passive filter has high signal transmission loss, and the passband edge thereof is not sharp, which results in a low selectivity Q.
In order to overcome the above problems of the passive filter, active filters are usually used in an MMIC. Such active filters can include a recursive filter and a transversal filter and a filter using an active inductor.
FIG. 1A shows the topology of a conventional first-order recursive filter 108. The filter 108 of FIG. 1A is described in an article entitled "Microwave tunable active filter design in MMIC technology using recursive filter," IEEE MMWMC Sym. Dig., pp. 105-108, 1995, by M.Delmond et al., which is incorporated herein in its entirety by reference. In the filter, one amplifier 110 is included in a reverse path to increase a signal-to-noise ratio (S/N). However, the filter of FIG. 1A has a drawback in that it has relatively low gain and low selectivity.
Accordingly, in order to increase the gain and selectivity of the filter, a filter in which two or more filters are connected in series has been suggested. FIG. 1B shows the topology of a second-order filter 116 in which two unit filters 112 and 114 are connected in series. The filter 116 of FIG. 1B is described in an article entitled "High-order monolithic active recursive filter based on multicellular approach," IEEE Int. Microwave Symp. Dig., pp. 623-626, 1996, by M. Delmond et al.
However, since the filter 116 of FIG. 1B includes a separate amplifier in each unit filter, it consumes a large amount of power and occupies a large amount of chip area. Also, the filter 116 uses a varactor diode for varying the passband of the filter. The impedance of the varactor changes according to a varactor control voltage, so the gain of the filter 116 varies according to a central frequency of the filter. FIGS. 2A and 2B illustrate such a phenomenon. Preferably, the gain of the filter must be constant even if the central frequency is changed, as shown in FIG. 2B. However, the gain of the filter of FIG. 1B decreases as the central frequency increases as shown in FIG. 2A.